This invention relates to vehicle door impact beams. Subsequent to enactment of Federal Motor Vehicle Safety Standard 214 (FMVSS214) which specifies side door strength requirements for vehicles, door impact beams of various designs have been proposed. Employees of the assignee herein have conceived and developed certain tubular beam designs which meet specifications of FMVSS214, while saving material, see especially U.S. Pat. Nos. 4,636,608 and 4,708,390 Subsequent developments are set forth in pending application Ser. No. 631,099, filed Dec. 19, 1990, and assigned to the assignee herein. Tapering the ends of the tubular beams has the advantages of 1) accommodating vehicle door curvature, 2) saving material, 3) lowering beam weight, and 4) potentially enabling the beam to be nestable (i.e. the material removed from the end of one beam can form the opposite end of the second beam, etc.).
The basic function of the impact beam is to minimize injury to persons in the vehicle during side collision. Side collision is characterized by an impact load or force applied generally horizontally, transverse &o the vehicle door, generally normal to the horizontal beam, thereby applying a bending moment to the impact beam which extends across the door within the confines of the inner and outer door shells. The beam should have moment capacity, at each location along its length, greater than the bending moment caused thereat by the specified impact load in the FMVSS 214 Standard.
The ability to increase the moment of inertia, thus increasing the moment capacity of the beam, without increasing beam weight, or preferably even with a decreased beam weight, would be particularly advantageous.
When the end portions of tubular beams are provided with a taper cut, the ends have only a fraction of the tube remaining there, i.e., a relatively small included angle with portions having an included angle of less than 180 degrees, and other portions having an included angle greater than 180 degrees. The "included angle" is that angle between two lines extending from the center line of the tubular beam to the taper edges on opposite sides of the center line, to indicate the degree of peripheral wall remaining.
If such a tapered beam structure is "edge loaded" relative to the remaining peripheral wall, i.e., has the impact force applied in a manner depicted in FIG. 7, the tube portion theoretically has a greater moment of inertia, i.e., I.sub.x =0.5I.sub.F, where I.sub.x is the moment of inertia at that point, and I.sub.F is the moment of inertia assuming the tube to be a full tube for included angles equal to 180 degrees. However, when a centrally located force is applied to the tubing structure, the reactive force at the tapered section is offset from the applied force causing a couple and hence twisting or torquing the tapered section, which may cause buckling. Therefore, prior taper cut beams, as in U.S. Pat. Nos. 4,636,608 and 4,708,390, have been mounted so that the impact force will be face applied relative to the end portions, as depicted in FIG. 8, thereby creating no torque on these end sections. However, the moment of inertia of these end portions is much less when face loaded, i.e., I.sub.x =0.095 I.sub.F for an included angle of 180 degrees.
When the included angle of the end portion is 180 degrees or greater, it is preferable to have the impact force applied in edge loading fashion (see FIG. 9), rather than face loading as in FIG. 10, because the moment of inertia is much greater as explained above, and moreover the force does not create an off center torque.
In consideration of these factors, a novel tubular impact beam was conceived to increase the moment capacity, or if desired to maintain the prior moment capacity while lengthening the taper cut.